Throughout the development of the computer industry, there has been an ongoing need in the art for fast and inexpensive storage means for large quantities of digital data. One of the least expensive means for storage of such data is on magnetic tape. However, a difficulty with magnetic tape is that it must be stored on reels and that, therefore, a portion of a tape to be accessed must first be unwound from the reel on which it is stored, thus limiting access. However, even given that the particular application in which magnetic tape is to be used does not require fast access it may and frequently does involve frequent stops and starts of the tape drive within a given file when, for example, a host computer requests the reading or writing of a record from the tape. Accordingly, it has been an aim of the art to provide a magnetic tape drive which can stop and start the tape within a very short period of time. Those skilled in the art will recognize that rapid stopping and starting of any mechanical system requires overcoming of inertial loads imposed by the movement of the masses to be stopped and started, i.e., the reels of tape themselves as well as associated motors and possibly other hardware. The most common prior art practice for overcoming the considerable inertial loads imposed by rather massive reels of tape, has been to "decouple" the portion of the tape in the vicinity of the read/write head from the bulk of the tape carried on the reels by providing plural vacuum columns or "buffers" or swing arms disposed between the reels and on either side of the read/write head whereby the only portion of the tape which must be accelerated quickly is that small fraction in the immediate vicinity of the read/write head. A quantity of tape could be inserted into or drawn out of a vacuum buffer or a loop taken up on the swing arm while the reels themselves came up to speed.
Concomitant with the use of vacuum buffers to decouple the bulk of the mass of the tape from that portion of the tape in the vicinity of the read/write head came development of elaborate control circuitry for varying the speeds of the two motors typically used to drive the tape supply and take-up reels (or, more conventionally, the "file" and "machine" reels). It will be appreciated, of course, that as tape is unwound from one reel onto the other their relative rotational speeds must vary if the linear speed of the tape past the read/write head is to remain constant, as is required by all presently used data coding systems. Numerous prior art patents show such vacuum buffers on either side of the read/write head and additionally show circuitry used to control the relative speeds of the two reels. See, e.g., U.S. Pat. Nos. 3,713,606 to Van Pelt et al; 3,984,065 to Boseti et al; and 3,648,134 to Audeh et al.
As mentioned above, all presently used tape data coding schemes involve constant speed of the tape past the read/write head. Typical prior art methods involve use of a comparatively heavy capstan in contact with the tape so as to provide a "flywheel effect" to its motion whereby transients in the speed of the tape are damped out by the mass of the flywheel capstan. Usually the capstan is moved into contact with the tape when it is desired that the tape move. In this way the capstan itself need not be stopped or started at either end of a read or write operation.
Later practice, as shown by the Van Pelt et al patent is to use a low-inertia, high performance capstan started and stopped with the tape. Clearly this causes restraints on the capstan design; typically a vacuum capstan is used to ensure good contact between tape and capstan.
More recently it has been realized as desirable that the vacuum buffers be eliminated so as to simplify the mechanical construction of the tape drive and to make it smaller in overall size. To this end more elaborate control schemes have been devised which allow the machine and file reels to be individually powered by servo motors and the tape to be directly connected therebetween. In order that the write operation not be delayed by the length of time required for the tape to come up to speed, a write command having been given, a solid state buffer memory can be provided to contain the data written during this period of time. Such a memory would amount typically to a sequential memory, data being fed in at one end by the host computer and written out of the other end onto the tape, the length of time required for data to pass from one end of the memory to the other being equal to or greater than that required to bring the tape up to speed.
It will be appreciated that in the prior art vacuum buffered tape drives, sensors could be provided within each of the vacuum columns to sense the precise position of the tape loop within the column so as to provide an exact indication of the relative speeds of the two reels and which could be used to provide a signal to correct the speed of the reels. In the more modern unbuffered (or direct coupled) tape drives, since there are no vacuum buffers, either the servo drives of the two reels must be very accurately controlled to keep the tension on the tape sufficiently low that it does not break, or other tension control means, such as a resiliently loaded arm with a idler reel mounted thereon, must be provided. Myriads of prior art references have addressed the problem of the precise control of the servo motors powering the two reels but none have achieved complete success. This lack of success is due to a variety of factors which complicate the design of such circuitry. For example, the variables which must be taken into account in such a system include the thickness of the tape, brush and bearing wear in each of the servo motors, windage (i.e. wind resistance) on the reels, which spin at comparatively high speeds, thermal variation in the speed of the motors as the tape drive is operated over a long period of time, drift of the analog circuit elements typically used in the servo loop circuits used to drive the reels, manufacturing tolerances and the like.
Numerous prior art expedients have been proposed in attempts to solve these problems. For example, U.S. Pat. No. 3,746,286 to Dennis et al shows provision of an arm over which the tape is passed in its passage between the two reels. The arm rotates on the shaft of a potentiometer, the resistance of which is, therefore, indicative of the tension on the tape. However, this apparatus involves undesirable mechanical complexity and inertia in the movement of the arm and furthermore, is insensitive to the thickness of the tape as well as the dimensional variations from one reel hub to the next.
The same objection applies to the scheme shown in U.S. Pat. No. 3,764,087 to Pannanen et al which shows a tachometer mounted on the shaft of one of the reels which can be used to generate a signal which in turn can be used to control the speed of that reel at a constant rate. It turns out that tape thickness can vary by as much as 10% and this can result in an error of as much as 6% in the speed of the tape when the Pannanen et al approach is used. An improvement on the Pannanen et al approach is disclosed in U.S. Pat. No. 3,984,868 to Ragle et al in which the gaps remaining between blocks of data recorded on a tape are measured to provide indication of the speed of the tape. Clearly the accuracy of the speed signal derived from the length of these gaps is limited by the accuracy to which their recordation can be controlled.
Still other prior art approaches involve the measurement of the tension on the tape, such as is shown in U.S. Pat. No. 3,910,527 to Buhler et al, or the physical measurement of the radius of the tape stored on a reel at any given time such as is proposed in U.S. Pat. No. 4,015,799 to Koski et al. These involve the clear drawbacks that mechanical sensors which are inevitably subject to physical variations as well as temperature drift must be provided.
One of the more promising prior art approaches is shown in U.S. Pat. No. 4,125,881 to Eige et al. This patent discloses using tachometers on each of the two reel shafts, one providing one pulse per revolution and one providing a plurality of pulses per revolution. The signals output thereby can be used to derive signals the relative accuracy of which is not affected by such matters as temperature drift, windage, motor brush and bearing wear and the like. However, the Eige et al disclosure still requires that a complex and expensive mechanical tension sensor be provided. This is used to control amplifiers in the servo loops and hence is essential to the Eige et al motion control scheme. Finally, the Eige et al scheme provides only a relative indication of the position of the tape, not an absolute position sensing apparatus. Accordingly, if for some reason both motors should vary together the Eige et al control scheme would not be able to detect this. Clearly it would be preferable to have an absolute position indication available at all times which would greatly simplify the control of the motion of the tape.
Accordingly, the need remains in the art for an improved tape drive which does not require mechanical sensors of stored tape radius, tape tension or the like and which does not involve mechanical tape buffer schemes, while providing increased performance in a physically diminutive package.